In various applications, it is desirable to be able to precisely determine the amount of current that is flowing through a load. For example, in motor vehicles, it is frequently desirable to determine a load current that is flowing through an oxygen sensor heater, as an associated oxygen sensor needs to be maintained at a desired temperature in order to optimally perform. Typically, a resistance value of the heater may be determined from a battery voltage level and a current level flowing through the heater. The temperature of the heater can then be determined by utilizing the resistance value and a known temperature coefficient for the heater. The temperature of the heater is then controlled, typically by pulse width modulating a switch or driver to adjust an average current and power of the heater.
With reference to FIG. 1, a system 100 that includes a microcontroller 10 has implemented a discrete sense resistor R1 to measure a current drawn by a heater (load) H. As is shown, the resistor R1 is coupled between a source of a field-effect transistor (FET) M1 and ground and a drain of the FET M1 is coupled to a low-side of the load H, whose high-side is coupled to a positive terminal of battery VBAT. The voltage signal across the resistor R1 is differentially coupled to an amplifier AV1, via a circuit network that includes resistors R2 and R3 and capacitor C1. The amplifier AV1 amplifies the voltage dropped across the resistor R1 to a range that can be accurately sampled by an analog-to-digital converter (ADC) located within the microcontroller 10.
An output of the amplifier AV1 provides a voltage that is measured by the ADC. The microcontroller 10 provides a pulse width modulated (PWM) signal to gate drive circuit 12 responsive to the voltage signal provided by the amplifier AV1. The gate drive circuit 12 provides a control signal to a gate of the FET M1 responsive to the PWM signal. One drawback of this design is that the requirements on the resistor R1 are fairly stringent, due to the high currents involved, when driving an oxygen sensor heater. For example, the resistor R1 is typically designed to handle currents between about six to ten Amperes. In order to meet power and overall voltage drop requirements, the value of the resistor R1 has typically been selected to have a relatively small value, e.g., 25 milliOhms. In such an application, the absolute value of the resistor R1 must generally be tightly controlled, which, in turn, causes the resistor R1 to be relatively expensive.
Another drawback with the above-described design is that a relatively large number of input/output (I/O) pins are required to interface the discrete devices. This tends to increase the system circuit board size and complexity, which also results in additional cost. Furthermore, using the above-described approach, a temperature of the discrete FET M1 cannot be readily sensed, as it is not integrated and, thus, complete fault protection of the discrete FET M1 cannot be achieved.
What is needed is an integrated driver for a heater that can be readily integrated, allows for improved fault handling capability and that provides relatively accurate thermal and current sensing capability. It would also be desirable if the integrated driver reduced I/O pin count over prior designs.